Zenith Aircraft Builders and Flyers
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Temporarily loosing power on climb out after taxiing on a warm day - vapor lock?
Hi everybody,
I now had two issues with our Rotax 912 ULS powered CH 750 CruZer.
In both cased I taxied for quite a while, the ambient temperature was around 25 degC / 77degF.
The engine initially made full power but when I was about 100 - 200 ft AGL it shook violently and almost died. Each time I pulled back on power, shortly after it ran smooth again.
We are currently in Phase 1 testing with just over 6 hours on the aircraft and engine.
Since we are in Phase 1, I did numerous full powers climbs without as much as a hick up.
The problems only occurred after taxiing.
The second time the issue occurred, I also noticed that that fuel pressured dropped several times briefly to around 1.5 psi, triggering a low fuel pressure alert, even though I had the aux fuel pump turned on as well.
Right after shutdown, I measured the temperatures at the fuel flow sensor as well as the gascolator, to which the fuel return line is routed. Both were are at around 47 degC / 116 degF.
I am now thinking that vapor lock might be the issue and am planning to wrap the exhaust / insulate the fuel lines.
Has anybody else had this problem? Do you agree with my diagnosis or is there something else I should consider?
Oliver
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Replies to This Discussion
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Permalink Reply by Jim and Amelia Isaacs on
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Hi Oliver,
I agree with your suspicions. Are all your engine compartment fuel lines covered in fire sleeve already? If so that should be sufficient protection from heat. Although some do, Rotax does not advise wrapping the exhaust for several reasons. And although some do, mostly for cost or convenience, Rotax does not advise routing the return line to anywhere other than a vented fuel tank. This is best way to help cool the fuel, and more importantly, release any trapped air, which will rise to the top then out of the vented tank. Trapped air has nowhere to escape to the atmosphere when it arrives in a gascolator. It will rise to the top of the gascolator- and be pulled back into the line to the carbs by the fuel pump. A related question, is your electric boost pump located directly under the fuel tanks and pushing fuel forward to the engine-driven pump?
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Permalink Reply by Peter Eiberger on
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Are you running auto fuel or avgas? Auto fuel is far more prone to vapor lock than avgas and winter blend auto fuel is even more prone than summer blend.
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Permalink Reply by Oliver Reik
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Hello Jim, hello Peter,
It turned out that that having the fuel restrictor jet of the fuel return line installed right on top of the gascolator was apparently the problem. We bought the hoses already crimped as a kit, there was therefore no easy way to change this.
As a 'quick fix' we installed a few feet of hose between the restrictor jet and the gascolator, to give the returned fuel some time to dissolve any bubbles, caused by cavitation, due to the rapid pressure drop at the restrictor jet.
Once we made this change, we were not able to reproduce the problem anymore, no matter how hard we tried.
Having looked deeper into this topic, I however also realized that it is not acceptable to return the fuel into the gascolator anyway, even though this appears quite common.
As per Rotax is it mandatory that the fuel is returned all the way back into one of the wing tanks. The fwf fuel lines also need to be insulated, even if hoses with an integral fire sleeve are being used.
We already insulated the fuel lines and will now also route the return line into one of the tanks.
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Thanks for the follow up Oliver, so to alert any future reader, who sells that kit? It may help prevent someone else from unknowingly installing a hazardous design.
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